Plasma etching and related processes such as reactive ion etching are becoming increasingly important in the field of semiconductor device manufacture. In general, these processes involve the exposure of one or more wafers containing a number of semiconductor devices to a chemical atmosphere which has been ionized by the application of RF energy. The usual goal of such processes is to remove exposed portions of an underlying layer while leaving an overlying layer which is usually a patterned photoresist.
As the feature size of devices manufactured by these processes becomes smaller, it becomes increasingly necessary to accurately define the endpoint of the process; that is, the point at which the desired portions have been removed. One method of performing endpoint detection is generally referred to as laser endpoint and involves the illumination of a predetermined portion of the wafer with energy from a laser and the analysis of the reflected energy.
As is disclosed in U.S. Pat. Nos. 4,198,261 and 4,208,240, both issued to Gould, Inc., laser endpoint detection is fundamentally an interferometric technique. The multiple reflections caused by the several layers with different indices of refraction create interference fringes which can be counted as the thickness of the layers change. While this has been well known for some time, laser endpoint detection has been used to a limited extent because of the necessity of devoting space on the wafer to serve as the test area and of aligning the wafer so that the test area coincides with the spot illuminated by the laser. In order to make sense of the signal carried by the reflected energy, prior art laser endpoint detectors need a test area in which only the layer which is to be etched away is present, with no overlying photoresist or masking layers.
The requirement of a test area and the necessity of alignment have also prevented the use of laser endpoint in the more general sense of a process monitor. For instance, if several laser endpoint detectors were used, say one at the center of the wafer and one near the edge, it would be possible to monitor the uniformity of the process across the wafer and to make changes in the process conditions if necessary.
Finally, laser endpoint detection has the drawback of requiring an optical window in close proximity to the wafer. In the popular parallel plate-type plasma reactors, the window must be placed in one of the electrodes which is used to excite the plasma. This often results in a discontinuity in the RF field in the region of the window and a differential etch rate for the test region of the wafer.